Wavelength division multiplexed optical communication systems are know in which multiple optical signals, each having a different wavelength, are transmitted from a transmit node to a receive node over an optical communication path including an optical fiber. Each optical signal is typically modulated in accordance with a particular modulation format to carry data. In certain conventional WDM systems, optical signals are modulated in accordance with an on-off-keying (OOK) modulation format in which the optical signal is modulated to carry digital data. In particular, the presence of optical signal light may indicate a 1 bit, and the absence or reduced intensity of such light may indicate a 0 bit, such that each optical signal may carry data as a series of light pulses.
OOK transmitters and receivers may have a relatively simple design and are often inexpensive, but have a limited data transmission capacity. Accordingly, so-called advanced modulation formats have been employed that have higher associated data rates. One such modulation format is referred to as a quadrature phase shift keying (QPSK) modulation format. In accordance with the QPSK modulation format, the phase, as opposed to amplitude, is modulated to carry symbols of data, each symbol including a combination of two bits. Other high data rate modulation formats are known, such as 8-quadrature amplitude modulation (8QAM).
High modulation format transmitters and receivers, while facilitating transmission of optical signals with associated high data rates, are relatively complex and typically cost more than OOK transmitters. In addition, capacity requirements of a WDM optical communication system typically do not suddenly spike, but may gradually increase over time. Accordingly, at least during an initial deployment, not all optical signals may be required to have a maximum data rate.
Thus, when upgrading a WDM system to provide QPSK optical transmitters, for example, it may not be economical, nor necessary, to replace all the OOK transmitters in the system with a QPSK transmitter at one time. Rather, capacity may be increased by swapping out each OOK transmitter over time. In doing so, the system would transmit a WDM optical signal including both OOK and QPSK modulated optical signals along the same optical communication path, such as an optical fiber, for example.
OOK modulated optical signals, however, may induce phase noise in the QPSK modulated optical signals through cross-phase modulation (XPM). Such phase noise, may reduce the optical signal-to-noise ratio (OSNR) of the transmitted QPSK modulated optical signals, and cause errors when the data carried by the QPSK signals are detected at the receive node.
Thus, an optical communication system is needed that can be economically upgraded to have a greater data carrying capacity and does not suffer from data transmission errors.